The present invention relates to a piezoelectric drive having a deformation layer which is coupled on one side to a piezoelectric layer, which is provided, for the electrical power supply, with at least one pair of electrodes made of two electrodes of opposite polarities. Such a drive is particularly, although not exclusively, suitable for the adjustment of a microvalve, wherein the membrane of said valve forms the deformation layer of the piezoelectric drive. Microvalves and the associated drives are used in the field of microfluidics to control fluid or gas flows. Here, the microvalves usually adopt only an open or closed state, so they have an on/off function. For the control of a throughput quantity, such microvalves can be operated in cycles with short cycle times. The setting element of the microvalve is here formed by a membrane which, in case of appropriate displacement, opens or closes a valve channel. In conventional microvalves, the fluid path through the valve channel is normally interrupted in the nondisplaced state of the membrane. A microdrive is required for the displacement of the membrane.
In microtechnology, electrostatic and piezoelectric drives are used. Piezoelectric drives are based on the sufficiently-known piezoelectric principle, where the application of an electric voltage to a piezoceramic causes a change in the length of the material. When the so-called longitudinal effect is used, a deformation of the piezo material, which can be used as drive, occurs in a direction which is in agreement with the direction of the electric field between two electrodes of opposite polarities. So-called high-power piezoelectric materials (PZT), which exhibit a change in length, the size of which is relatively large given the material thickness, are based on this longitudinal effect, in which the polarization direction of the piezo material is in agreement with the electrical field direction. For example, to move a membrane using such a drive, the piezo material is firmly mounted on one side, and on the other side it is connected to the membrane.
In these applications, the achievable adjustment distance is approximately 1/1000 of the thickness of the piezo material. To achieve a mechanical lift of 1 μm, a piezo material thickness of at least 1 mm is consequently needed. If a larger mechanical lift is needed, the piezo material thicknesses increases accordingly, which has negative consequences for the use of these drives in microtechnology. Moreover, actuation voltages with a field strength of approximately 3000 V/mm are needed to supply the piezoelectric material, which, in microtechnological applications, can rapidly lead to undesired electrical flashover. The control devices needed for such actuation voltages are difficult to integrate in microtechnological solutions.
In EP 0 914 563 B1, a piezoelectrically actuated microvalve is described, which uses the mentioned longitudinal piezoelectric effect. In the process, a piezoelectric actuation device is coupled to the frame of a suspension device. The suspension device also carries a ram which closes a passage opening in the valve. When a voltage is applied to the piezoelectric actuation device, a change in the length of the piezo material occurs, which results in a deformation of the suspension device, which shifts the ram perpendicularly to the length change of the piezo material.
Piezoelectric materials moreover exhibit the so-called transverse effect which causes a deformation of the piezo material perpendicularly to the direction of the electrical field between the electrodes. When a voltage is applied to the piezo material, the latter contracts transversely to the field direction. The deformation achievable by the transverse effect is in all cases smaller than 1/1000 of the piezo material expansion.
DE 36 18 107 A1 discloses an ink writing head with piezoelectrically excitable membrane, where the drive makes use of the mentioned piezoelectric transverse effect. The drive of the membrane is implemented by a typical layer design. A metal supporting layer is attached to a ground substrate, and it extends over a fluid conveying channel opening. The metal supporting layer carries a polarized layer made of piezoceramic on the side facing away from the channel opening. The supporting layer acts as a ground electrode for the generation of the electrical field which excites the piezo material. On the opposite side of the piezoceramic, actuation electrodes are provided, which allow a locally-delimited activation of the piezo material. The required electrical field is thus generated between the ground electrode and the respective actuation electrode, which are positioned on opposite sides of the piezo material. To actuate the electrodes, a deformation of the piezoelectric ceramic layer and of the metal supporting layer attached thereto occurs, which functions as a membrane. Due to the special arrangement of the multiple actuation electrodes, a deformation of the supporting layer in the direction towards the piezo material can be achieved. However, for this purpose, field directions that are directed in the opposite direction of the polarization direction in the piezo material must be generated in certain regions. Operating a piezo material against the polarization direction leads, as experience has shown, to a depolarization of the piezo material, which in the medium term leads to a loss of the piezoelectric properties. Another problem is that the metal supporting layer is in direct contact with the fluid in the channel to be controlled, which can lead to corrosion of the supporting layer, and to electrical problems. Consequently, such a design is not desirable for many microtechnological applications.
U.S. Pat. No. 6,222,304 B1 discloses a shell transducer which can be designed, for example, to move a fluid or solid medium. An electroactive medium in the form of a piezoelectric layer is arranged, for example, above a buffer layer made of ZrO2 which should undergo deformation together with the electroactive medium. Current from two electrodes flows through the electro active medium.
From U.S. Pat. No. 5,255,016, a printer head for an ink jet printer is known. The printer head has vibrating plates which function as pumps for the ink. The vibrating plates are made of a piezoelectric material. Two electrodes are applied to a surface of the piezoelectric material.